(1) Field of the Invention
The present invention generally relates to a hydraulic brake control apparatus, and more particularly to a hydraulic brake control apparatus which appropriately controls a brake fluid pressure in a brake system of an automotive vehicle.
(2) Description of the Related Art
A hydraulic brake control apparatus having two pressure supplying units connected to wheel cylinders of the automotive vehicle is known. For example, Japanese Laid-Open Patent Application No. 4-243655 discloses such a hydraulic brake control apparatus.
In this hydraulic brake control apparatus, a master cylinder and a high-pressure supplying unit are provided as the two pressure supplying units. The master cylinder supplies a brake fluid pressure varied in response to a brake operating force on a brake pedal, to the wheel cylinders. The high-pressure supplying unit supplies a given brake fluid pressure, regardless of the brake operating force, to the wheel cylinders.
In the apparatus of the above-mentioned publication, the master cylinder is connected to the wheel cylinders via flow control valves, and the high-pressure supplying unit is connected to the wheel cylinders via flow control valves. When the high-pressure supplying unit operates normally, the flow of the brake fluid from the master cylinder to the wheel cylinders is inhibited by the flow control valves, and the brake fluid pressure from the high-pressure supplying unit is supplied to the wheel cylinders.
On the other hand, when a defect in the high-pressure supplying unit has occurred, the flow of the brake fluid from the high-pressure supplying unit to the wheel cylinders is inhibited by the flow control valves, and the flow of the brake fluid from the master cylinder to the wheel cylinders is allowed by the flow control valves. In this case, the brake fluid pressure, varied in response to the brake operating force on the brake pedal, is supplied by the master cylinder to the wheel cylinders.
According to the apparatus of the above-mentioned publication, it is possible to supply a proper brake fluid pressure from the high-pressure supplying unit to the wheel cylinders when the high-pressure supplying unit operates normally, and it is possible to supply a proper brake fluid pressure from the master cylinder to the wheel cylinders when the high-pressure supplying unit is defective. By utilizing the above apparatus, a fail-safe function of the brake system against a defect in the high-pressure supplying unit can be achieved.
If the brake fluid pressure is continuously supplied by the pressure supplying units when a defect in any of brake fluid paths between the pressure supplying units and the wheel cylinders has occurred, the brake fluid may leak out of the defective path. In order to avoid this, a brake fluid supply stopping control is carried out by a conventional brake system when the occurrence of the defect in the brake fluid paths is detected. When the brake fluid supply stopping control is carried out, the supply of the brake fluid pressure to the wheel cylinders through the defective path is stopped to prevent the leak of the brake fluid.
Generally, a hydraulic circuit in the brake system of the automotive vehicle is divided into two hydraulic circuit systems. When a defect in a brake fluid path of one hydraulic circuit system has occurred in the conventional brake system, the supply of the brake fluid pressure to the wheel cylinders through the defective path is stopped, and the supply of the brake fluid pressure to the wheel cylinders through brake fluid paths of the other hydraulic circuit system is maintained. Therefore, by utilizing the conventional brake system, a fail-safe function against a defect in one of the two hydraulic circuit systems is achieved, and a braking force on the vehicle is produced by using the other hydraulic circuit system in which no defect occurs.
However, if the supply of the brake fluid to the wheel cylinders through one hydraulic circuit system is stopped when a defect in one of the brake fluid paths of the hydraulic circuit system has occurred, it is difficult for the brake system to produce an adequate level of braking force on the vehicle by using only the brake fluid pressure supplied to the wheel cylinders through the normal hydraulic circuit system.
If the flow of the brake fluid between the defective path and the wheel cylinders is inhibited by suitably setting the flow control valves of the hydraulic circuit system when the occurrence of the defect in the hydraulic circuit system is detected, it is not necessary to stop the supply of the brake fluid pressure to the wheel cylinders through the defective hydraulic circuit system. It is desirable to provide a fail-safe brake system which effectively produces an adequately great braking force without causing the leak of the brake fluid when a defect in one of the brake fluid paths of the brake system has occurred.